The present invention relates generally to a process for carburizing a steel component to increase the surface hardness of the material. More particularly, in one form the present inventive process includes electroless nickel plating the outer surface of a martensitic stainless steel component prior to vacuum carburizing. Although the present invention was developed for processing components formed of stainless steel, certain applications extend outside of this field.
In the design and manufacture of steel components, there is often a need to modify properties of the material. It is well recognized that carburizing is a process suited for hardening the surface and sub-surface of the steel component. Carburizing can be broadly considered as either an atmospheric carburization process or a vacuum carburization process. In the vacuum carburization process, the component is heated to an elevated temperature within a carburizing furnace, and a carburizing gas is introduced into the environment so that carbon atoms are diffused into the surface and sub-surface of the steel material. The carbon content in the surface and near sub-surface of the component is increased while the carbon content within the core of the component remains unaltered. The characteristics of the component have thus been modified to provide a hardened outer surface surrounding an interior core.
In response to the continued demand for new goods and services, engineers and scientists are always seeking to enhance products through material selection and/or process development. Stainless steel is widely utilized in many components in a vast array of products. One stainless steel of interest is available under the tradename, Pyrowear 675. A known technique associated with carburizing the Pyrowear 675 component is to oxidize the surface of the component prior to exposure to the carburizing environment. The component is grit blasted and placed in an air furnace at a temperature of 1800° F. for about one hour to form an oxide on its surface. Upon the component being subjected to the carburizing environment, the oxidized surface facilitates the absorption of carbon by the material.
In a carburizing process the time and temperature that the material is subjected to while in the carburizing environment will determine the surface hardness, case depth, hardness profile, and carbide microstructure of the hardened portion of the material. In the prior method discussed above, after carburization the Pyrowear 675 material is annealed, hardened, annealed, hardened, stabilized in a deep freeze, tempered, brought to room temperature, and then tempered again. With reference to FIG. 1, there is illustrated a prior heat treat cycle for carburizing and hardening the Pyrowear 675 material. Further, with reference to FIG. 2, there is illustrated a hardness profile for a carburized Pyrowear 675 component that was processed with the heat treat cycle set forth in FIG. 1.
While there are many prior processes for carburizing steel components, there remains a need for additional development in this area. In furtherance of this need, the present invention provides a novel and non-obvious means for carburizing steel.